1. Technical Field
The present invention relates to a linear measuring arrangement for measuring the relative position of two objects, having a scale in a housing and a scanning unit, which can be displaced relative to the unit comprised of the scale and the housing, and a mounting piece.
2. Background Information
Such linear measuring arrangements, such as described in DE 29 29 989 A1, for example, are used for measuring lengths, as well as paths, and are employed in particular in processing machinery for measuring the relative movement of a tool in relation to a workpiece to be processed, in coordinate-measuring machinery, and increasingly also in the semiconductor industry.
A scale, which has been placed in a housing for protection against environmental effects, is employed here as the measurement representation. Temperature changes inside the housing cause a change of the length of the scale as a function of the material of the scale. These changes in length lead to measurement errors.
To keep such temperature changes as small as possible, it is proposed in DE 101 64 404 A1 to regulate the temperature in the interior of the encapsulated linear measuring arrangement by passing a controlled cooling medium through it.
By this it is intended to achieve that the scale takes on the temperature of the machine tool. It is disadvantageous here that temperature changes generated by electrical components of the linear measuring arrangement itself are not taken into consideration.
The demands made on linear measuring arrangements continue to increase, higher resolution, as well as increased accuracy and reproducibility of the position measurement continue to be demanded. At the same time a compact mechanical construction, as well as the simple and interference-free generation of measurement values and measurement value transmissions, should be provided.
These requirements demand an encapsulated linear measuring arrangement with a protectively housed scale. High resolution requires an increasingly reduced scanning distance, which is constant over the entire length to be measured. This is achieved on the one hand by a relatively small scanning distance of less than 100 μm, and on the other hand by guiding the scanning unit on the scale and/or on the housing wherein, for the undisturbed precise guidance of the scanning unit, the mounting piece is coupled to the scanning unit by a coupling which is rigid only in the measuring direction. In all other directions, this coupling allows a movement of the mounting piece without an adverse effect on precise guidance and the movement of the scanning unit in the measuring direction.
A compact construction and an interference-free generation of measured values and measured value transmission require the integration of increasingly more electrical components in the scanning unit itself. Therefore sensor chips are increasingly employed for this, on which an array of scanning sensors is arranged, for example light-sensitive detectors in connection with a scale which can be scanned optically-electrically, as well as signal processors, such as A/D converters, amplifiers, micro-processors and interfaces, for example. An increased heat generation in the scanning unit results from this which, because of the required short scanning distance, heats the scale to an incomparatively large extent. This heating leads to changes in the length of the scale and to measurement inaccuracies.